In the field of disposable sanitary hygiene articles at the end of the 1980s, absorbent material in granules was introduced for forming their absorbent structure, which was previously composed of just cellulose fibers.
The absorbent material in granules that is normally used in this type of disposable hygiene products is composed of super-absorbent polymers capable of absorbing and retaining large quantities of liquids.
Super Absorbent Polymers (SAP) can, in turn, have granules of different sizes and shapes, according to the different production methods.
The production of an absorbent structure can be carried out in several ways. One of the most common methods is carried out by depositing and/or mixing the absorbent polymers granules on a strip composed of synthetic and/or natural fibers that can be absorbent, for example cellulose fibers.
The desire of all manufacturers of disposable sanitary hygiene products is to be able to concentrate the absorbent material in areas where it is most used, i.e. to create discontinuous applications of the absorbent polymer granules, always maintaining control of the weight and the geometric parameters of the application, such as the length and width of the applied dose and pitch of application between the various doses; so as to achieve a higher quality product that, at the same time, results in a saving in terms of cost, and creates products with a lower environmental impact as they are made with a smaller quantity of materials.
Systems are known in the art that are able to apply a quantity of absorbent polymer material in granules in a discontinuous manner suitable for producing absorbent structures for disposable sanitary articles. According to the known, well described embodiments, for example, in patent document EP 1 621 165 A1, such systems consist of a rotary dispensing cylinder provided, on its outer surface, with a plurality of slots and/or recesses arranged at the deposition area, and having dimensions that ensure the deposition of the correct quantity of absorbent material in granules or SAP. The rotary dispensing cylinder is normally placed at the bottom of a reservoir from which it picks up the material in granules. Subsequently, by rotating, the cylinder carries the slots and/or recesses, filled with SAP, into a second area or discharge area where it releases it. The discharge area is normally diametrically opposite to the loading area, and the material discharged from the dispensing cylinder can be deposited on a moving strip.
The inventors have observed that apparatus such as the one described above have numerous limitations and/or problems such as, for example, the control of the quantity of the absorbent material in granules applied to each product, which can be done only in an indirect manner.
Indeed, the weight of material introduced in the production method of the absorbent structure can only be determined with the aid of the apparent density of the material in granules, in other words, the quantity of SAP that is required for depositing on the absorbent products is defined solely by the volume of the slots and/or the recesses present on the outer surface of the dispensing roller, which are intended to be filled by said absorbent polymeric material in granules.
It should be recalled that the density or volumic mass of a body is defined as the ratio between the mass of a body and its volume.
The definition of density provided above refers to a quantity of solid and homogeneous matter, i.e. without internal voids. This value is also known as the real or absolute density, since it only takes into account the volume of the solid fraction.
For solid materials with closed cavities, with open cavities or spongy structures, or for granular matter contained in the recipients, such as sand, grains or as in our case absorbent material in granules, the concept of apparent density of a body is introduced, which is calculated in a manner formally analogous to the absolute density, but takes into account the total volume occupied by the solid, thus its external dimensions, including the empty spaces present inside.
This type of control has very important limitations related, specifically, to the variability of the apparent density. Indeed, the apparent density changes according to the pressure head variation in granular material present in the loading reservoir, the variation in the environmental conditions (temperature, humidity) and not least to the variation in size of the granules themselves, which of course can vary between one supplier and another, but also to the variation in production batches from the same supplier.
Furthermore, the slots and/or recesses have difficulty filling and emptying their contents with increasing velocities of the production machines. To overcome this difficulty, dispensing cylinders are provided with gripping and releasing means of the absorbent material in granules. To do this, the bottom of the slots and/or recesses of the dispensing cylinder are rendered permeable to air, and connected to a source of sub-atmospheric pressure during the loading phase, and subsequently, to a high pressure pneumatic source during the phase of expulsion or discharge.
This system, while solving the problem of filling and emptying of the dispensing roller in high-velocity production lines, in turn, generates new problems such as, for example, increasing the complexity and consequently the cost of the dispensing roller and the increase in maintenance operation costs due to the need to always keep the air-permeable area of the slots and recesses clean.
In addition to what has already been said, the problem linked to the change in production size, and therefore the specifications, of the absorbent structure, must not be forgotten. Indeed, it is evident that each format of the absorbent structure is also characterized, in addition to its dimensions, by the quantity and distribution of the absorbent material in granules. Therefore, each of the said formats of the absorbent structure requires its own specific dispensing roller, which, obviously, must be replaced when the size of the product to be produced changes.